As a mechanism for changing valve timing of an internal combustion engine, a hydraulically-driven variable valve timing mechanism is known. In the hydraulically-driven variable valve timing mechanism, a rotor fixed to a distal end of a camshaft is accommodated in a housing fixed to a sprocket. A plurality of vanes protruding in the radial direction are provided in this rotor. On the other hand, accommodation chambers accommodating these vanes, respectively, are provided in the housing. As a result, each accommodation chamber is divided into a phase advancing hydraulic chamber and a phase retarding hydraulic chamber by the vane.
In the internal combustion engine provided with the variable valve timing mechanism configured as above, the rotor is rotated in the housing by adjusting a hydraulic pressure in the phase advancing hydraulic chamber and the phase retarding hydraulic chamber to change relative rotational phases of the rotor and the camshafts with respect to the sprocket. As a result, valve timing of the intake valve or the exhaust valve is changed.
In order to realize the valve timing suitable for engine starting, the relative rotational phases of the rotor and the camshaft with respect to the sprocket need to be fixed to relative rotational phases suitable for the engine starting. However, since a stable hydraulic pressure cannot be ensured at the engine starting, it is difficult to hold the relative rotational phase of the rotor with respect to the sprocket by the hydraulic pressures in the phase advancing hydraulic chamber and the phase retarding hydraulic chamber. Thus, a lock mechanism for holding the relative rotational phase of the rotor with respect to the sprocket to a lock phase, which is a relative rotational phase suitable for the engine starting is provided. When the internal combustion engine is stopped, the rotor is fixed to the lock phase by the lock mechanism. The lock mechanism includes a lock pin and a lock hole engaged with the lock pin and restricts the relative rotation motion of the rotor with respect to the sprocket by inserting the lock pin into the lock hole.
At the engine starting, the rotor is preferably fixed to the lock phase by the lock mechanism, but if the rotor cannot be fixed to the lock phase when the engine is stopped, the rotor might not be fixed to the lock phase when the engine is started. In such a case, since the valve timing at the engine starting becomes unstable, the engine starting might not be able to be completed or it might take time to start the engine.
In order to cope with the above, however, in an internal combustion engine described in Patent Document 1, if the rotor is not fixed to the lock phase at the engine starting, the hydraulic pressure is used to rotate the rotor to the lock phase to fix the rotor to the lock phase by the lock mechanism.
Moreover, in an internal combustion engine described in Patent Document 2, a plurality of stepped portions with different depths are provided on the bottom face of the lock hole, and these stepped portions are arranged so that the depths thereof become gradually deeper toward the lock phase. If the camshaft rotates, a positive torque for rotating the rotor and the camshaft in a direction to retard the valve timing and a negative torque for rotating the rotor and the camshaft in a direction to advance the valve timing alternately act on the rotor and the camshaft with opening/closing of the valve by a cam. If the positive torque and the negative torque act on the rotor and the camshaft at the engine starting when the hydraulic pressures in the phase advancing hydraulic chamber and the phase retarding hydraulic chamber have not sufficiently risen, the rotor rotates alternately in the phase advancing direction and the phase retarding direction in the housing. If the rotor rotates in the housing as above, the lock pin sequentially fits in the plurality of stepped portions with different depths provided on the lock hole in the lock mechanism, whereby the rotor gradually rotates toward the lock phase, and finally, the rotor reaches the lock phase where the rotor is fixed by the lock mechanism. That is, in the internal combustion engine described in Patent Document 2, the lock mechanism is provided with a ratchet mechanism, and the rotor is rotated to the lock phase at the engine starting by the action of this ratchet function.